Changes Around Us Class 6 NCERT Science: Full Concepts & Solved Practice

What are Reversible and Irreversible Changes?

To study the changes around us class 6 ncert topics systematically, scientists divide changes into two main categories: reversible and irreversible changes.

1. Reversible Changes: A change is called reversible if we can reverse the process to get back the original substance in its initial form. Think of folding a sheet of paper to make a toy aeroplane. If you unfold it, you get the same sheet of paper back. Similarly, stretching a rubber band or melting an ice cube are physical changes that can be easily undone. The composition of the substance remains identical; only its shape, size, or physical state changes temporarily.

1. Irreversible Changes: If a change cannot be undone to recover the original materials, it is called an irreversible change. For example, if you burn a sheet of paper, it turns to ash and smoke. You cannot collect the ash and convert it back into paper. Baking a chapati from dough, the growth of a small plant into a tree, or the ripening of a mango are all permanent transformations that cannot be reversed.

Process of Expansion and Contraction (How Iron Rims are Fitted)

1. Selecting the Sizes — An iron rim is made slightly smaller in diameter than the wooden wheel on which it needs to be mounted so that it fits tightly once cooled.
2. Heating the Iron Rim (Expansion) — The iron rim is heated strongly over a fire. On heating, metals undergo a process called expansion, which means they swell and increase in size, making the rim slightly larger.
3. Mounting the Rim — Because the heated rim has expanded and become larger, it easily slips onto the wooden wheel.
4. Cooling with Water (Contraction) — Cold water is immediately poured over the hot iron rim. This sudden cooling causes the metal to undergo contraction (shrink back to its original smaller size), securing a tight, immovable grip on the wooden wheel.

Crucial Comparisons to Remember

• Melting of wax vs. Burning of a candle: Melting wax is a reversible physical change because you can solidify the melted wax back into its solid form. Burning a candle is irreversible because the wick burns into ash, and some wax escapes as gaseous carbon dioxide and water vapor.
• Dissolving sugar in water is a reversible change because you can recover the sugar crystals back by heating the solution and evaporating the water.
• Most physical changes (like folding, tearing, stretching, or freezing) tend to be reversible, whereas chemical changes (like cooking, burning, digesting, or rusting) are almost always irreversible.

CBSE Exam Secret: Is Tearing Paper Reversible?

Many students lose marks in exams by writing that 'tearing a sheet of paper' is a reversible change because 'it is still paper'.

Here is the exact way to write your answer for CBSE evaluations:
Tearing paper is technically a physical change because no new chemical substance is formed. However, it is an irreversible change because you cannot physically merge the torn pieces back together to form the original single undivided sheet without using glue (which introduces a new material!). Always read the exam question carefully to see if they are asking for reversible vs irreversible or physical vs chemical.

Practice Questions with Solutions

• Q: A thick coating of a paste of Plaster of Paris (POP) is applied over the bandage on a fractured bone. It becomes hard on drying. Can this change in POP be reversed? A: Step 1: Understand the reaction. Plaster of Paris (POP) reacts with water to undergo a chemical hydration process. Step 2: Note the physical transformation. The wet paste absorbs water and sets into a completely new, hard solid structure (gypsum). Step 3: Determine reversibility. Because this chemical reaction creates a new substance with different properties, adding heat or water again will not turn the hard mass back into the original dry POP powder. Final answer: No, the hardening of Plaster of Paris is a chemical, irreversible change that cannot be undone.
• Q: A bag of cement lying in the open gets wet due to rain during the night. The next day, the sun shines brightly. Do you think the changes which have occurred in the cement could be reversed? A: Step 1: Analyze the situation. Wetting cement initiates a chemical process called hydration. Step 2: Analyze the next step. Sunlight dries the wet cement, turning it into a hard, stone-like block. Step 3: Assess the possibility of reversing it. The chemical composition of cement has changed permanently during hydration. It cannot be converted back into soft dry cement powder by any physical method. Final answer: No, the chemical changes that occurred in the cement due to water action are irreversible.
• Q: Classify the following changes as reversible or irreversible: (a) Dissolving salt in water, (b) Baking a cake, (c) Melting of chocolate, (d) Rusting of an iron nail. A: Step 1: Analyze each case separately. Step 2: (a) Dissolving salt in water: We can evaporate the water to get the salt crystals back. So, it is reversible. Step 3: (b) Baking a cake: Raw ingredients undergo chemical reactions under heat to form a cake. We cannot extract flour, eggs, and sugar back. So, it is irreversible. Step 4: (c) Melting of chocolate: Melted chocolate can be placed in a refrigerator to solidify back to its original state. So, it is reversible. Step 5: (d) Rusting of iron: Iron reacts with oxygen and moisture to form iron oxide (rust), which is a completely new compound. So, it is irreversible. Final answer: Reversible changes: (a) Dissolving salt in water, (c) Melting of chocolate. Irreversible changes: (b) Baking a cake, (d) Rusting of an iron nail.
• Q: How does heating affect an iron key when we try to fit it into a tight lock, or a blacksmith making tools? A: Step 1: Identify the underlying scientific principle. When metal objects are heated, they absorb thermal energy and expand in volume. Step 2: Detail what happens on heating. A blacksmith heats an iron piece until it becomes red-hot. At this point, it expands and becomes soft, making it easier to beat into desired shapes. Step 3: Detail what happens during cooling. Once the shape is formed, it is cooled down, causing the iron to contract and harden back into its original solid structure. Final answer: Heating causes the metal tools to expand (increase in size) and become soft, which is a reversible process as cooling contracts them back to their original size and hardness.

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